The present invention relates generally to a method of increasing the capacity of a CDMA (Code Division Multiple Access) network and to central and/or remote units for implementing the method, the network being a wireless local loop network, for example, or a mobile radio network.
To give one example that is not limiting on the invention, the network is one that conforms to the ANSI IS-95 Standard and uses PN codes on the uplink (from the fixed or mobile terminal to the base station) and a combination of Walsh and PN codes on the downlink (from the base station to the fixed or mobile terminal). The network can equally well be one that conforms to the UMTS Standard currently being drawn up by the ETSI, in which case it uses a combination of Walsh and Gold (or Kasami) codes on the uplink (from the fixed or mobile terminal to the base station) and a combination of Walsh and Gold codes on the downlink (from the base station to the fixed or mobile terminal).
The invention applies to any uplink or downlink of a CDMA system which uses, in combination, for each call, a code referred to as an internal spreading code, of the Walsh-Hadamard, etc. type, and a code referred to as an external xe2x80x9cscrambling codexe2x80x9d, of the PN, Gold, Kasami, etc. type. The terms xe2x80x9cinternalxe2x80x9d and xe2x80x9cexternalxe2x80x9d are used here to distinguish between two types of code or sequence, for example the Walsh and PN types (see, for example, U.S. Pat. No. 5, 103, 459). The code is referred to as xe2x80x9cinternalxe2x80x9d if the various codes of the set of codes to which it belongs can be used in their entirety within the same cell. Conversely, the code is referred to as xe2x80x9cexternalxe2x80x9d if the various codes of the set of codes to which it belongs are not intended a priori to be used within the same cell.
In a CDMA network, a code xe2x80x9cchipxe2x80x9d Tc constitutes the basic level coding symbol employed and Ts=N.Tc is the duration of a basic symbol of the call, typically one bit (binary digit), N being the ratio between the total spectrum required for N calls and the spectrum required for a single call. N is commonly referred to as the xe2x80x9cprocessing gainxe2x80x9d of the CDMA code.
In the CDMA technique used in the context of the invention for the uplink or downlink concerned, on transmission, the symbols of each call are multiplied by two codes from two sets of CDMA codes. For example, these two codes are a sequence referred to as Walsh-Hadamard sequence and a PN sequence, as proposed in U.S. Pat. No. 5, 103, 459, which is hereby incorporated in this application by way of reference. Such sequences are defined by a series of bits having a much higher bit rate than the symbols of the call. This is reflected in the spreading of each call across a much wider band than theoretically required to transmit the symbols of the call in the baseband. On reception, the coding sequence(s) used on transmission are reproduced locally in the receiver in order to separate each call spread on transmission over a band corresponding to N times the band required for each call.
Thus, in simple terms, each CDMA call is defined by the continuous occupation in time (Ts) of a band of frequencies equal to N times the frequency band occupied by the call in the baseband. In the prior art, the total number of CDMA calls that can be set up simultaneously is therefore limited by this factor N.
A document issued by the ETSI (European Telecommunication Standard Institute) under the title xe2x80x9cETSI UMTS-L1 98-208xe2x80x9d (which is hereby incorporated in this application by way of reference) proposes to use different external PN scrambling codes in the same cell of the mobile radio network in order to increase the capacity of the cell. The above document indicates that the drawback of this solution is that interference in a cell between orthogonal internal codes (in this case Walsh-Hadamard sequences) combined with different PN scrambling codes is greater than the interference between orthogonal codes if only one external scrambling code is used, because the property of orthogonality is lost. The above document therefore proposes to use intelligent or adaptive antennas and to assign the various scrambling codes to respective different portions of the cell, beam forming assuring that there is very little interference between the different codes.
Thus the prior art technique described in the above ETSI document requires intelligent antennas. This prior art solution therefore makes it essential to employ an antenna technology that is extremely difficult to implement. An intelligent antenna, whose coverage beam varies with time, requires particularly high processing intelligence to cover all areas including mobile terminals which can set up a call using a dimension of the beam which is a function of the density of the mobiles and also to prevent beams overlapping each other in order to prevent the problem of interference between codes, as mentioned in the above ETSI document. Furthermore, there is no increase in the capacity of the cell in real terms, but rather the creation of sub-cells within each cell. In reality, the capacity remains the same for each sub-cell.
The document xe2x80x9cThe cdma2000 ITU-R RTTxe2x80x94Candidate Submission to US TG8/1xe2x80x94TR45.5""s ITU-R RTT candidate submission approved on May 15, 1998xe2x80x9d published by the I.T.U (INTERNATIONAL TELECOMMUNICATION UNION), mentions, in particular in Appendix Q, quasi-orthogonal functions for increasing the capacity of the cells of a CDMA network. However, the above document does not propose any solution to the problem of interference between calls, which reduces the technical viability of the proposed solution.
The present invention therefore aims to remedy the aforementioned disadvantages by providing a method of increasing by M the theoretical maximum number N of calls that can be set up on the same CDMA link and providing an effective solution to the problem of interference between calls.
To this end, in a method in accordance with the invention of increasing by M, where M is any integer greater than or equal to one, the number N, where N is any integer greater than or equal to 2, of calls set up between a central unit and remote units of a code division multiple access network, N first group calls plus M second group calls being able to be set up simultaneously in this way and each using an internal spreading code and an external scrambling code:
first interference caused by first group calls in each second group call is synthesized,
said first synthesized interference caused by said first group calls is subtracted from each second group call,
first interference caused by second group calls in each first group call is synthesized, and
said first synthesized interference caused by said second group calls is subtracted from each first group call.
Iteratively, for any value of P greater than or equal to 2:
(1) in a first sub-step, a respective Pth interference that first group calls cause in each second group call is synthesized using decisions on values of symbols of said first group calls taken during a (Pxe2x88x921)th second sub-step, after which said Pth interference is subtracted from the respective first group call, and symbol values of each second group call are decided on after subtracting said Pth interference from each second group call, and
(2) in a second sub-step, a respective Pth interference caused by second group calls in each first group call is synthesized using decisions on symbol values of said second group calls taken during the first sub-step (1), after which said Pth interference is subtracted from the respective first group call, and symbol values of each first group call are decided on after subtracting said Pth interference from each first group call.
For example, the N first group calls use respective Walsh-Hadamard orthogonal internal codes and the same external scrambling code of the set of external scrambling codes (PN code, Gold code, Kasami code), and the M second group calls use a sub-set of M orthogonal Walsh-Hadamard internal codes used by the first group calls and another external scrambling code of the set of external scrambling codes (PN code, Gold code, Kasami code) different to that used for the first group.
In another embodiment of the invention, the N first group calls use respective mutually orthogonal internal codes and the same external scrambling code of the set of external scrambling codes (PN code, Gold code, Kasami code) and the M second group calls use respective other mutually orthogonal internal codes which are not orthogonal to the internal codes of the first group calls and an external scrambling code of the set of scrambling codes (PN code, Gold code, Kasami code) identical to the external code used by the first group calls.
The invention also provides a receiver unit of a code division multiple access telecommunication network, N first group calls plus M second group calls being able to be set up simultaneously in this way and each using an internal spreading code and an external scrambling code, the receiver unit comprising:
a first synthesizer for synthesizing a first interference caused by first group calls in each second group call,
a subtractor for subtracting said first interference caused by all the first group calls from each second group call,
another synthesizer for synthesizing a first interference caused by all second group calls in each first group call, and
another subtractor for subtracting said first interference caused by all the second group calls from each first group call.
The unit advantageously comprises, where P is any integer greater than or equal to 2:
for a first sub-step, a Pth synthesizer for synthesizing a respective Pth interference that first group calls cause in each second group call using decisions on values of symbols of said first group calls taken during a (Pxe2x88x921)th second sub-step, a subtractor for subtracting said Pth interference from the respective second group call, and decision means for deciding on symbol values of each second group call after subtracting said Pth interference from each second group call, and
for a second sub-step, a Pth synthesizer for synthesizing a respective Pth interference caused by all second group calls in each first group call using decisions on symbol values of said second group calls taken during the first sub-step, a subtractor for subtracting said Pth interference from the respective first group call, and decision means for deciding on symbol values of each first group call after subtracting said Pth interference from each first group call.